(i) Field of the Invention
The present invention relates to a process for the treatment of perfluorinated gases, in particular perfluorocarbons, as well as to hydrofluorocarbons, for the purpose of destroying them, as well as to a treatment apparatus implementing such a process.
The reduction of gaseous emissions caused by human activity, which are believed to be able to contribute to global warming of the climate by an increase in the greenhouse effect, is currently a major preoccupation of international proceedings. The drafting of directives and recommendations within each country, harmonized within the framework of worldwide conventions, is presently at a relatively advanced stage. The States involved could, in the relatively near future, as a next step, adopt mandatory statutory measures.
Currently, the attention of industrial concerns and governments is essentially focused on the most well-known greenhouse-gas species, namely carbon dioxide (CO.sub.2), the amount of which discharged and the concentration of which in the atmosphere are by far the greatest.
However, other gases may also make an equally important contribution to possible warming of the climate, although the volumes of these which are discharged are smaller than those of CO.sub.2 by several orders of magnitude. These are the perfluorinated compounds (PFC) and hydrofluorocarbon (HFC) compounds, since these gases have lifetimes in the atmosphere and absorptions in the infrared which give rise to a "global warming capability" which may be several orders of magnitude greater than that of carbon dioxide.
Thus, carbon tetrafluoride (CF.sub.4) only exists in the Earth's atmosphere at an average concentration of approximately one part per million (ppmv) compared to 300 to 400 ppmv in the case of CO.sub.2. However, its lifetime in the atmosphere is estimated to be approximately 50,000 years compared to 100 in the case of CO.sub.2, while its absorption in the infrared is considerably higher.
The situation with regard to these gases is also special because of their widespread use in the semiconductor industry. They are used for plasma etching the patterns in integrated circuits and, above all, for cleaning, also using a plasma, thin-film deposition reactors. They are not completely consumed in these processes and give rise to residual emissions which are far from being negligible.
Compared to many other sectors, the microelectronics industry was aware very early on of the problem of gaseous-effluent reprocessing. There are several reasons for this state of affairs, one of which is the spectacularly dangerous nature of some of the substances involved--very high toxicity, highly corrosive and damaging nature with regard to the respiratory tracts, inflammability and explosiveness. It is therefore quite understandable that this industry is also the first to be concerned about limiting the discharges of perfluorinated and hydrofluorocarbon greenhouse gases which represent a potential hazard of a completely different nature, these being in general not very reactive and having a low to zero toxicity.
Furthermore, certain producers of these gases have chosen as of now to take initiatives which are restrictive vis-a-vis their customers, anticipating the predictable outcome of the official regulatory framework, for example by no longer supplying users who do not take the appropriate measures to destroy or recover a minimum amount (for example 80%) of the unreacted C.sub.2 F.sub.6 emanating from process machines.
Consequently, integrated-circuit manufacturers are currently seeking to find a quick solution to the problem of reducing emissions of C.sub.2 F.sub.6 and, more generally, of all the PFCs and HFCs.
The idea of an intermediate step consisting in preferably using, in processes, molecules which, among this range, have the lowest global warming capabilities is completely illusory.
This is because the choice of a molecule for a particular process, etching or reactor cleaning, is governed by its specific properties. Only very few conceivable replacements exist, especially with regard to cleaning which today is the application involving the highest consumption of active gases and for which C.sub.2 F.sub.6, C.sub.3 F.sub.8 and NF.sub.3 appear almost uncircumventable for reasons of rapidity of the process.
It is therefore in all cases necessary to envisage either destroying or recovering the perfluorinated and hydrofluorocarbon gases which have not reacted in processes and which would otherwise be discharged into the atmosphere.
(ii) Description of Related Art
Several solutions exist which, a priori, are conceivable for destroying PFCs and HFCs, these having reached different levels of maturity from the technical and commercial standpoint.
At the present time, only pyrolytic destruction using a burner has been achieved on an industrial scale using equipment available on the market.
In these systems, the molecule to be destroyed is thermally decomposed by means of the heat supplied by the combustion of natural gas and/or hydrogen. After decomposition, the fluorine is in the form of very reactive acidic chemical species, in particular HF, which is then very easy to destroy by reaction with an alkaline aqueous solution.
This technique is widely used at the present time. However, it has many disadvantages.
Like any process of combustion in air, it generates nitrogen oxides which are themselves harmful to the environment and which form, or will form, the subject of specific regulatory limitations with regard to their emissions. Moreover, if the combustible gas is natural gas, large quantities of CO.sub.2 will be generated.
The potential of global warming of the atmosphere thus produced is very much less than that of the fluorinated gas which is destroyed. However, many authors think that this CO.sub.2 emission is a dissuasive element in the dissemination of technology, in particular because the regulations regarding carbon dioxide are changing more rapidly than those regarding PFCs and HFCs.
In fact, natural gas is only used in part, or even not at all, for destroying PFC and HFC compounds since, in order to achieve high destruction yields (95 to 99%), especially in the case of the most stable compounds such as SF.sub.6 and CF.sub.4, it is necessary to burn hydrogen, and to do so in large quantities. Of course, this raises problems of cost and, to an even greater extent, of safety, greatly limiting the advantage to potential users.
In order to help to overcome these drawbacks, it has been envisaged to treat PFCs and HFCs by thermochemical decomposition over a solid, or even by pure and simple complete reaction with this solid. However, these molecules are highly stable, which singularly restricts the range of conceivable materials, and the necessary temperatures in general remain high.
The only promising results have been achieved using catalysts based on extremely expensive noble metals. More common solids capable of providing realistic destruction efficiencies, as is the case with other molecules for which satisfactory commercial systems exist, have not yet been identified.
Faced with the difficulty posed by the destruction of perfluorinated compounds and hydrofluorocarbons, it is possible to envisage another way of reducing the emissions thereof, namely the recovery of these substances after separation from the effluent mixture emitted by the process reactors. In fact, this technique is not completely concurrent with destruction. Indeed, the technological choices may be influenced by certain factors relating, for example, to regulatory constraints or to the professional practices in the country in question.
Thus, in the United States, semiconductor fabrication plants generally include a centralized system for collecting and treating the gaseous effluents, while in Japan the practice is moving more in the direction of small treatment systems close to the point of emission.
However, the known recovery technologies are profitable only in the case of large outputs and are particularly suitable for centralized approaches.
Moreover, it is presently not known how to realize destruction systems in the case of high outputs. The concepts of destruction and recovery are complementary and must be developed in parallel.
There is therefore, whatever the circumstances, a need for destruction systems which are more effective, less expensive and less restrictive than hydrogen burners.